Multi-layer golf balls contain a core, which may comprise one or more layers of solid material (i.e., a "solid" core), or one or more layers of solid material encompassing a liquid therein (a "liquid" core), a mantle, and a cover. Optionally, an elastic winding may also be used to form a layer surrounding the center, as discussed below, to provide desired playing characteristics. Such balls are known as "wound" balls. The terms "center" or "ball center," as used herein, refer to a solid and/or liquid mass around which at least a mantle and cover are formed. The terms "core" or "ball core," as used herein, include a center having one or more layers and a mantle formed of one or more layers. The mantle, also known as an intermediate layer when a cover is present, is disposed between the center and the cover, typically in concentric fashion, with the cover being the outermost portion of the ball. The term "cover," as used herein, means a dimpled layer disposed concentrically about the core.
A number of compositions are known and used in various methods for the manufacture of the core of multi-layer golf balls as discussed below. Some of these processes use curing systems that combine a rubber, curing agent and a peroxide, while others use peroxides or other curing agents having different reactivities or reaction rates to prepare golf ball mantles or other elastomeric compositions.
For example, U.S. Pat. No. 3,989,568 discloses a process and resultant composition for covering golf balls with polyurethane by selecting a system of at least three reactants having different rates of reaction to partially cure cover half shells that can be disposed about a golf ball core and then fully cured to form the ball cover. The system of reactants includes materials having different reaction rates, such as two polyurethane prepolymers and a single curing agent or one prepolymer and two curing agents.
U.S. Pat. Nos. 4,546,980 and 4,692,497 disclose a process of making a solid golf ball having an unexpected increase in initial velocity capability using at least two free radical initiators having different reactivities. The process involves mixing a polymer and metal salt of unsaturated carboxylic acid together at a high temperature, reducing the temperature below the stable temperatures of the free radical initiators, adding the initiators to the polymer mixture and mixing them therein, shaping pieces of mixture slightly larger than the desired golf ball product, and curing them at high temperature to form the center of the ball product.
U.S. Pat. No. 4,650,193 discloses a two-piece golf ball having a core with an outer layer, and a cover, and methods for production of the ball including the use of one or more conventional peroxide, free radical initiator catalysts to crosslink the core and a sulfur-bearing material to modify peroxide crosslinking in the outer layer.
U.S. Pat. No. 4,781,383 discloses a solid three-piece golf ball made by covering a core, which has inner and outer layers, with a shell. The outer layer is prepared by using a metal mold to prepare two hemispherical premolded products, which are used to cover the previously molded inner layer of the core. The outer layer is then cured around the inner layer by heating the entire core before adding the shell.
U.S. Pat. No. 5,006,297 discloses a process for preparing a polyurethane covered golf ball by allowing the polyurethane cover stock material to partially cure in an open mold and subsequently molding a golf ball product, then curing the golf ball product before molding to impart dimples. This process provides curing both prior to and after placing cores in the mold, where the pre-curing is described as essential to the formation of a good smooth ball.
U.S. Pat. No. 5,019,319 similarly discloses a one-piece golf ball or core of a multi-layer ball formed from a rubber composition of a base rubber component modified with an unsaturated carboxylate group. The composition may include a mixture of two or more free radical releasing agents having different reactivity to enhance co-crosslinking.
U.S. Pat. No. 5,116,060 discloses golf balls having an improved coefficient of restitution provided by polymerizing one or more elastomers in the presence of at least one metallic salt of an unsaturated carboxylic acid and a free radical initiator of t-amyl peroxide or blends thereof with one or more organic peroxides having a similar half life.
U.S. Pat. No. 5,688,595 discloses a multi-piece solid golf ball having a core with at least two layers of an inner sphere layer and a surrounding layer, and a cover having outer and inner cover layers. The surrounding layer may be formed by molding half shells in a semi-vulcanized state, enclosing the inner sphere, and compression molding again to yield the core.
It is desirable to use hemispherical shells containing thermoset material(s) to form one or more mantle layers about a golf ball center, although this often results in poor centering of the mantle and other problems in processing because thermoset materials are difficult to work with before they have been crosslinked. Moreover, the polymers typically used in such shells tend to have a memory that urges the polymer back to its earlier or original shape, which necessitates rapid compression molding to crosslink the polymer as soon as the shells are formed.
None of these references, however, recognizes the need for a curing system for use in a golf ball product that permits a partial cure of the mantle prior to assembly to facilitate assembly about a center, while at the same time permitting a subsequent cure without requiring the addition of initiators or other curing agents after the partial cure. There is thus a need for an improved method for manufacturing multi-layer golf balls that avoids the disadvantages present in current curing methods, particularly when using shells to form one or more mantle layers containing thermoset material about a center. It is therefore desired to find a new composition and method for manufacturing a portion of a mantle in a golf ball core using a first and second heat curing, as this will advantageously improve the symmetrical formation of the core in golf balls, facilitate bonding between the shells, and facilitate automated assembly, which greatly reduces production costs.